1. Field of the Invention
This invention relates to a flat plate collimator which is able to condense diffuse light into parallel rays, and a backlight system comprising the above collimator; in particular, it relates to a flat plate collimator with improved display characteristics in liquid crystal displays, and a backlight system.
2. Description of the Related Art
A transition is in progress from CRT displays to liquid crystal displays as the display means used in word processors, personal computers, and other office automation equipment.
However, due to the birefringence and optical rotation arising from the alignment of liquid crystals, the viewing angle characteristics of the display contrast and display colors of liquid crystal displays are inferior to those of CRT displays. Consequently there have been attempts to improve the viewing angle characteristics by incorporating phase difference plates (Japanese Patent no. 2565644 publication), and it has been proposed that attempts be made to reduce the adverse effects of the birefringence of liquid crystals by controlling the liquid crystal alignment; however, at present the display characteristics have not reached the level of CRT displays.
One cause of the degradation of display characteristics in liquid crystal displays is the fact that the light incident on the liquid crystal cells is not completely parallel rays, but is diffuse light having a certain spreading. Light incident on a liquid crystal cell at various angles other than the perpendicular may cause degraded display quality due to coloration and other problems. Hence if light in completely parallel rays can be made incident on liquid crystal cells, declines in display characteristics caused by the liquid crystal birefringence would not occur, and the display characteristics could be improved. Here there are in the prior art no members used in liquid crystal displays to change diffuse light into parallel rays, that is, no flat plate collimators, and at most, prism sheets and the like which condense diffuse light are in use in some areas.
The above prism sheets have poor parallel properties, require complex manufacturing processes, and are expensive. Further, it is difficult to form such members as thin films, and there is the danger that use of such members will cause loss of brightness. In no cases were such prism sheets sufficient for practical use.
This invention was devised in light of the above problems. An object of this invention is to provide a flat plate collimator, not existing in the prior art, which can be manufactured easily and inexpensively.
Another object of this invention is to provide a high-quality backlight system capable of irradiation of parallel rays, and which, when applied to a liquid crystal display in particular, can improve the display contrast, viewing-angle characteristics, and other display characteristics.
That is, in order to resolve the above-described problems, this invention provides the following collimators and backlight systems.
(1) A collimator comprises a cholesteric liquid crystal layer exhibiting a selective reflected wavelength range of wavelengths xcex1 to xcex2 (xcex1 less than xcex2) for perpendicularly-incident light, and with respect to the wavelength xcex0 of maximum intensity in the emission spectrum of the light source used in combination, satisfies the relation xcex0 less than xcex1.
(2) A collimator comprises a dielectric multilayer film exhibiting a selective reflected wavelength range of wavelengths xcex1 to xcex2 (xcex1 less than xcex2) for perpendicularly-incident light, and with respect to the wavelength xcex0 of maximum intensity in the emission spectrum of the light source used in combination, satisfies the relation xcex0 less than xcex1.
(3) A backlight system comprises a light-reflecting layer, a light source exhibiting an emission spectrum having emission maximum at wavelength xcex0, a light-diffusing layer, and the collimator described in (1) above.
(4) A backlight system comprises a light-reflecting layer, a light source exhibiting an emission spectrum having emission maximum at wavelength xcex0, a light-diffusing layer, and the collimator described in (2) above.
By means of the collimator of this invention, even when light at wavelengths xcex1 to xcex2 (xcex1 less than xcex2) is incident on the collimator, due to the selective reflection effect of the cholesteric liquid crystal layer or dielectric multilayer film comprised by the collimator, light in the wavelengths xcex1 to xcex2 (xcex1 less than xcex2) is reflected. On the other hand, when light at the maximum-emission wavelength xcex0 of the light source, which is not included in the above selective-reflection wavelength band, is incident perpendicularly on the cholesteric liquid crystal layer or dielectric multilayer film, the light is emitted from the collimator as parallel rays. However, in the selective-reflection wavelength band of the cholesteric liquid crystal layer or dielectric multilayer film, light is shifted to shorter wavelengths depending on the angle of incidence xcex1. As a result, the wavelength xcex0 is included in the selective-reflection wavelength band shifted to shorter wavelengths, and when light at wavelength xcex0 is incident on the cholesteric liquid crystal layer or dielectric multilayer film at an angle of incidence xcex1(xcex1 greater than 0), it is reflected due to the selective-reflection effect of the cholesteric liquid crystal layer or dielectric multilayer film. Hence only light at wavelength xcex0 from the above light source, incident normal to the above cholesteric liquid crystal layer or dielectric multilayer film, can pass through the above cholesteric liquid crystal layer or dielectric multilayer film, and only parallel rays are emitted.
The selective-reflection wavelength band and other optical characteristics of the above cholesteric liquid crystal layer or dielectric multilayer film can easily be adjusted by selecting materials and controlling the alignment, so that the above collimator can easily be constructed. And, by selecting materials and controlling the alignment, the above-described function can be realized even when the thickness of the cholesteric liquid crystal layer or dielectric multilayer film is made thin, so that demands for thinness can be satisfied.
In this invention, xe2x80x9cperpendicular incidencexe2x80x9d signifies incidence perpendicular to the collimator. The angle of incidence a refers to the angle of incidence of light relative to the above-mentioned helical axis, and may be defined for example as a in FIG. 2. Further, in this invention xe2x80x9ccholesteric liquid crystal layerxe2x80x9d refers to a cholesteric liquid crystal layer the helical axis of which is substantially in the normal direction to the cholesteric liquid crystal layer, that is, the Grandjean alignment.
In the backlight system of this invention, light which cannot pass through the above cholesteric liquid crystal layer or dielectric multilayer film and is reflected has its direction of propagation disordered by the light-diffusing layer, and is again incident on the above cholesteric liquid crystal layer or dielectric multilayer film. Or, after being reflected by the light-reflecting layer, light is again incident on the above cholesteric liquid crystal layer or dielectric multilayer film. By repeating this process, light from the light source is formed into parallel rays by the collimator of his invention.
Hence by means of the backlight system of this invention, parallel rays can be irradiated, and when applied to a liquid crystal display, the backlight system can contribute greatly to improvement of the display contrast and broadening of viewing angles.